Treesearch

This volume contains in part papers presented at the Symposium on Monitoring Bird Population Trends by Point Counts, which was held November 6-7, 1991, in Beltsville, Md., in response to the need for standardization of methods to monitor bird populations by point counts. Data from various investigators working under a wide variety of conditions are presented, and various aspects of point count methodology are examined. Point counts of birds are the most widely used quantitative method and involve an observer recording birds from a single point for a standardized time period. Statistical...

A set of 255 counts was compiled for 13 points using 10-minute periods subtallied at 3 and 6 minutes. The data from each point were subsampled using combinations of count periods, numbers, and schedules to compare the effectiveness of these different regimes at per point coverage. Interspecifically, detection frequencies differed in level and pattern as a function of count period length. The complex nature of detection frequencies is discussed in relation to density and to species-specific activity patterns. Short count periods (3 minutes) are more sensitive to changes in activity levels...

Estimation of avian densities from point counts requires information about the distance at which birds can be detected by the observer. Detection distances also are important for designing the spacing of point counts in a regional sampling scheme. We examined the relationship between distance and detectability for forest songbirds in northern Wisconsin. Like previous investigators, we found that some birds can be heard from much greater distances than others. Within the same species, some individuals (or the same individual under different circumstances) can be heard from greater distances...

To examine sample size requirements and optimum allocation of effort in point count sampling of bottomland hardwood forests, we computed minimum sample sizes from variation recorded during 82 point counts (May 7-May 16, 1992) from three localities containing three habitat types across three regions of the Mississippi Alluvial Valley (MAV). Also, we estimated the effect of increasing the number of points or visits by comparing results of 150 four-minute point counts obtained from each of four stands on Delta Experimental Forest (DEF) during May 8-May 21, 1991 and May 30-June 12, 1992. For...

Comparisons of bird abundances among years or among habitats assume that the rates at which birds are detected and counted are constant within species. We use point count data collected in forests of the Mid-Atlantic states to estimate detection probabilities for Neotropical migrant bird species as a function of count length. For some species, significant differences existed among years or observers in both the probability of detecting the species and in the rate at which individuals are counted. We demonstrate the consequence that variability in species' detection probabilities can have...

We report results of a point count survey of breeding birds on Hoosier National Forest in Indiana. We determined sample size requirements to detect differences in means and the effects of count duration and plot size on individual detection rates. Sample size requirements ranged from 100 to >1000 points with Type I and II error rates of <0.1 and 0.2. Sample size was inversely related to species abundance (r = -0.38, P < 0.01). Counting efficiency was maximized at a count duration <6 minutes with a travel time of 8 minutes, but differences were slight for most travel times. Unlimited-radius...

On a medium site in northern California, a mostly shrub community was treated by two manual release techniques and by two herbicides, to study its development in both a natural (control) and treated condition. Survival and growth of planted ponderosa pine seedlings were quantified for 8 to 11 years after initial treatment applications. Treatments included manual release in a 4-foot radius around pine seedlings one, two, and three times; grubbing the entire one-seventh acre plot two times; applying 2, 4-D and Velpar herbicides to the entire plot one time; and a control. Data are presented...

A mathematical policy model of oak woodlands is presented. The model illustrates the policy uncertainties that exist in the management of oak woodlands. These uncertainties include: (1) selection of a policy criterion function, (2) woodland dynamics, (3) initial and final state of the woodland stock. The paper provides a review of each of the uncertainty issues. The final section of the paper describes a modeling approach that can be developed to assist policy makers in evaluating alternative oak woodland policy actions.

We used standardized transect techniques to count Marbled Murrelets and other seabird species at sea from a boat and from a low-flying light aircraft along the length of the Oregon coast. The focus of effort was on vessel surveys of the central Oregon coast. In both years, Marbled Murrelets were most abundant in central Oregon, between Cascade Head and Cape Arago. They were concentrated much closer to shore in 1992 than in 1993. Different distribution patterns in the two years was likely a consequence of El Niño oceanographic conditions which severely impacted Oregon’s seabirds in 1993....

Marbled Murrelets occur in Puget Sound marine habitats in relatively low numbers. The rates of occurrence of murrelets on censuses within marine habitats is generally low. Further analysis is required to determine if low occurrence rates are a general function of the movement of birds, or their consistent occurrence on particular censuses and not on others. Qualitative data indicate that Marbled Murrelet abundance in Puget Sound is now lower than earlier this century. Such long-term information is unavailable for Marbled Murrelets along the Pacific Ocean coast of Washington. Census data...